How can i fix my may 05 renault megane convertible?

Just bought a very clean, mint condition, renault megane 1.6 petrol convertible, just picked it up 2 hrs ago, and on the way home with it, the engine management light came on , and the car started to shudder, so i pulled over and turned the car off for a few minutes , and then turned it on again, when i turned it on again there was a slight clunk, started to drive off again and the management light came back on and the shudder. I have rang the dealer were i bought it, and they are coming tomorrow to see the problem, i am not going to drive it, but i just wanted to know could this be a serious issue and should i be thinking about my money back.